The present invention relates to a technique for recording, reproducing and erasing an information signal with respect to an opto-magnetic recording medium having a magnetic anisotropy perpendicular to its surface.
Nowadays, there has been developed an apparatus in which information can be recorded on and read out of a recording medium by means of a laser beam, said recording medium being formed by a magnetic film of two-phase amorphous alloy such as GdCo, GdFe, TbFe and DyFe and three-phase amorphous alloy such as GdTbFe. In case of recording, the magnetic film has been previously magnetized perpendicularly, for instance downward and the laser beam modulated with an information signal to be recorded is projected onto a given portion of the film so as to heat the related portion near a Curie point while the related portion is subjected to an external biassing magnetic field directing upward. Then the direction of magnetization in the relevant portion is inverted. Since the information can be recorded perpendicularly in the magnetic film, a very high recording density can be attained. In case of reproducing the information thus recorded in the recording medium, the laser beam is projected via a polarizer onto the medium and the reflected or transmitted laser beam is received by a photodetector via an analyzer. The polarization surface of the linearly polarized laser beam is rotated to some extent due to a magnetic Kerr effect or a magnetic Faraday effect in a direction depending upon the direction of magnetization at the read out portion of recording medium. By detecting the rotational direction of the polarization plane of the detected laser beam, the information can be read out. In such an opto-magnetic recording medium, not only the recording and reproducing, but also erasing can be effected and thus, the information can be rewritten just like in an ordinary magnetic recording medium such as magnetic tape, magnetic disc and floppy disc.
In one of the recording and/or reproducing method of the kind mentioned above, a laser light beam is projected onto a thin film of MnBi or rare earth element-iron group having Kerr effect, or rare earth garnet having Faraday effect, and a direction of magnetization in the recording medium is inverted by means of heat generated by the laser beam irradiation so as to record information. Such a method is superior to a method in which the information is permanently recorded on a surface of metal film by means of a series of holes or pits, because in the former method, the information can be rewritten repeatedly. Such a method can record and/or reproduce the information signal and is termed as opto-magnetic recording and/or reproducing method. However, in the known opto-magnetic method all the information recorded on the record medium must be erased at once by placing the record medium in a strong magnetic field and by subjecting the record medium to heat, if any. By means of such an erasing method it is difficult to erase a part of the recorded information and to record new information in the erased portion.
FIG. 1 is a schematic diagram showing one embodiment of the known method for recording information on an opto-magnetic record medium. A laser beam 1, modulated in accordance with an information signal to be recorded, is converged into a light spot 3 by means of an objective lens 2. Then the light spot 3 impinges on a record medium 6 which comprises a thin film 5 having an opto-magnetic effect applied on a substrate 4. When a temperature of the irradiated portion on the thin film 5 is increased higher than the Curie point, the direction of magnetization 7 in this portion is inversed by an external magnetic field 8 having a magnetizing direction opposite to that of a surrounding magnetization by a demagnetizing field produced by the surrounding magnetization, and then a series of bits 9 are formed on the record medium 6 corresponding to the information signal to be recorded.
Hereinafter, bit means a region which records a minimum unit of information and, if the information is analog data such as a video or an audio signal, the bit corresponds to a so-called pit.
The reproduction of the information recorded in this manner is performed by projecting a linearly polarized beam onto the record medium 6 and then detecting a rotation of a polarized surface of a light reflected from the record medium 6.
Next, a method of erasing the information recorded in this manner will be described. In case of erasing all the information recorded on the record medium 6, the record medium 6 on which the bits 9 are formed as shown in FIG. 2A is heated above the Curie point by means of a suitable heating means. At the same time, the external magnetic field 8 having a comparatively weak magnetic field or having a magnetic field above a coercive force Hc in case of no heating is applied to the record medium 6 so as to arrange the magnetizing direction on the record medium 6 the same as that of the external magnetic field 8 as shown in FIG. 2B, so that all the bits 9 recorded on the record medium 6 can be erased completely.
Moreover, in case of erasing the information recorded on the record medium 6 partly at random, the laser beam 1 is projected onto a given restricted portion of the bits as shown in FIG. 3A to heat the irradiated portion locally, and then the magnetization of the relevant portion is forcibly oriented in the same direction as that of the external magnetic field 8 having a magnetic force above that of a demagnetizing field Hd produced by the neighboring magnetization, so that the relevant portion of the bits 9 can be erased selectively as shown in FIG. 3B.
However, in an actual information recording and/or reproducing apparatus, it is convenient to perform the erasing and rewriting operations at the same time. A known method of performing the erasing and rewriting at the same time will be described with reference to FIG. 4. In FIG. 4, when the record medium 6 is moved in an the direction of the arrow direction, an erasing beam 11 scans a series of bits 9 to erase the information recorded on the record medium 6 in the same manner as shown in FIG. 3. It should be noted that the magnetizing direction of the bit is from back to front with respect to a plane of the drawing, and the external magnetic field Hex is applied in the direction opposite to that mentioned above. A numeral 12 shows a recording beam. In case of focussing the erasing beam 11 and recording beam 12 by means of only one objective lens, it is not possible to make the distance between these two beams large because of their aberrations, etc. The direction of the magnetic fields applied at the erasing beam spot and the recording beam spot have to be opposite to each other, and therefore if these magnetic fields are applied simultaneously under such a condition that the distance between the erasing beam and the recording beam is close, the effects of these beams would be cancel each other, so that it is not possible to record new information by the recording beam 12 while erasing the previous information by the erasing beam 11.
FIG. 5 shows another known method in which the erasing and recording are performed at the same time by varying the direction of the single magnetic field, while the single beam is projected on the record medium continuously. In this case, the irradiated portion of the record medium is heated above the Curie point by the laser beam 13. In this embodiment, when a recording signal is not supplied, the external magnetic field Hex having an amount larger than the coercive force Hc is applied. When the recording signal corresponding to the information to be recorded is supplied, the direction of the external magnetic field Hex is inverted at the same time, and then a recording operation can be performed by applying the external magnetic field having an amount smaller than the coercive force Hc at a room temperature. However, since it is difficult to make the inductance of a magnetic field generating coil small for carrying out the erasing and recording operations and since the inversion of the magnetic field is delayed if a coil having a large inductance is used, it is not possible to change the erasing and the recording operations rapidly by inverting the magnetic field in response to the information to be recorded which is supplied at a high speed.
As described above in detail, in a known opto-magnetic recording and/or reproducing method, it is not possible or at least very difficult to effect the erasing and rewriting operations at the same time.